1. Field of the Invention
The present invention relates to a method for producing a resin mold that is used to produce a screen of, for example, a rear projection video apparatus.
2. Description of the Prior Art:
A transmission-type screen of, for example, a rear video projector is generally formed of a Fresnel lens and a lenticular lens. Also, the material forming such transmission-type screen contains a diffusion agent, such as SiO.sub.2, CaCO.sub.3. Al.sub.2 O.sub.3, TiO.sub.2, BaSO.sub.4, ZnO, glass beads, an organic-system diffusion agent or the like so as to have a suitable diffusion property for the light which is incident thereon. The Fresnel lens has the function to refract diverged light from a projection lens toward a viewer. This Fresnel lens is formed of a methacrylic resin plate one side of which is formed a lens surface on which a number of concentric grooves of V-shaped configuration are formed. The other side is formed as a non-lens surface. Generally, a so-called hot press molding method is used to manufacture such a Fresnel lens screen.
According to the hot press molding method, the Fresnel lens screen is produced as follows. First, a metal mold is prepared. This metal mold is produced by cutting a copper plate or an aluminum alloy plate having a thickness of 10 to 20 mm by means of an NC (numerical control)-type cutting machine in such a manner that a plurality of concentric grooves of V configuation are formed on one side surface so as to form a pattern opposite to that of the desired Fresnel lens. Next, a metal plate whose surface is polished to a mirror surface is prepared. Then, a methacrylic resin plate having a thickness of 2 to 4 mm is sandwiched between the thus made metal mold and the metal plate with the mirror surface, and it is then heated and then molded with pressure so as to produce the Fresnel lens screen.
In another method an extrusion-process using a molding roller is employed to produce the lenticular lens screen.
When the above-mentioned transmission-type screen is mass-produced according to the hot press molding process, a number of metal molds which are cut are needed. Further, it takes 150 to 200 hours to finish one metal mold by the cutting process. Also, considering the yield in the cutting process and the life of the metal mold (the number of lens screens that can be made per one press) and so on, the cost of the metal mold amounts to a very large percentage of the manufacturing costs of the above-mentioned transmission-type screen.